Why Black Holes Are More Than They Seem

Black holes are notorious for gobbling up, well, everything. They're icons of destruction, ruthless voids, ambivalent abysses from which nothing can return — at least, according to pop culture. But black holes have another side: Astrophysicists have seen powerful jets, sometimes millions of light-years long, shooting out of supermassive black holes – including the one at the center of our own galaxy. So today, we're getting to know the other side of black holes, and the powerful role they may play in creating and shaping the cosmos. Read more https://journals.aps.org/prd/abstract/10.1103/PhysRevD.99.124013 about the Blandford-Znajek process. Got other cosmic curiosities? Email us at [email protected] mailto:[email protected]. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:15Speaker 00

  You're listening to Short Wave from NPR.

• 0:20Regina Barber?

  In pop culture, black holes have developed this reputation for gobbling things up. Being these points in the universe where all matter, even light, is inescapably sucked up into this extraordinarily dense black void.

• 0:33Speaker 02

  They're often seen as sort of cosmic vacuum cleaners,

• 0:38Speaker 02

  just sucking in all the material, gas, and stars that stray close.

• 0:44Regina Barber?

  That's Priyam Bhatta Natarajan, an astrophysicist at Yale University who focuses on black holes, specifically how extremely large ones came to be. And Priya says these supermassive black holes, like the one in the center of our own Milky Way galaxy, are more than just cosmic vacuum cleaners.

• 1:03Regina Barber?

  Because when black holes eat material immediately around them, they create this bright disk, like a glowing donut. But...

• 1:11Speaker 02

  What is counterintuitive is that we do see very powerful jets of material that are actually expelled from them as well.

• 1:22Regina Barber?

  Basically, black holes are really messy eaters, so not all the dust and gas they eat make it down the hatch. And for a supermassive black hole, this can look like beams of white-hot plasma and radiation shooting out of that glowing donut just outside the event horizon that makes up the edge of the black hole. Sometimes these beams are millions of light years long.

• 1:44Regina Barber?

  Roger Blandford says you can think of these black hole jets carrying this massive amount of energy, kind of like nuclear power.

• 1:51Speaker 03

  Of course, they can be famously destructive, but also it can be a source of power in a nuclear reactor.

• 1:58Regina Barber?

  Roger's an astrophysicist and a professor at Stanford University. In the 1970s, when he and his friend Roman Znajek were at the University of Cambridge over in the UK, they started to look at how these black hole jets were created. And they came up with a hypothesis for how these jets were powered, but it would take some time for all these pieces to come together to see if this explanation held true.

• 2:23Regina Barber?

  So today on the show, a look at the most energetic objects in the universe: supermassive black hole jets. What are they, how they might be created, and what new images can tell us about these mysterious objects? I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR.

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• 4:09Regina Barber?

  OK, so funny thing about black hole jets: they were first imaged before astronomers could even agree on what a galaxy was. Hebra Curtis is one of the astronomers at the center of that debate, and he's the one who first identified a jet in 1918. At first, it seemed like a strange, bright streak in the cosmos.

• 4:28Speaker 03

  And he saw what he called a curious straight ray, and that was an optical photograph of a jet.

• 4:36Regina Barber?

  The ray was coming out of a fuzzy thing named M87, we now know to be an enormous galaxy 50 million light-years from Earth. And M87...

• 4:46Speaker 02

  harbors a supermassive black hole that's like 6 billion times the mass of the sun, and it has a huge jet that's coming out that extends all the way and whose origin we can actually link to the black hole itself.

• 5:03Regina Barber?

  Fast forward to the early 70s when Roger was starting graduate school and astronomy was buzzing with new discoveries. Humans had landed on the moon in 1969. The first black hole was confirmed to exist in 1971 after decades of mathematical theory. And astronomers were looking at the really bright centers of galaxies in our universe.

• 5:27Regina Barber?

  The centers of these galaxies have extremely massive black holes. And some also seem to have these bright streaks or jets of energy coming from the middle, just like the one Curtis saw coming out of M87 decades earlier.

• 5:40Speaker 03

  So I was watching my colleagues and friends learn about these jets using their wonderful radio telescopes, which have got more and more wonderful as the years have rolled by.

• 5:52Regina Barber?

  And these jets, they found, that were coming from these black holes were huge, four times as long as they were wide. These blazing streaks were stretching far past the width of the galaxies they lived in.

• 6:03Speaker 03

  This was a truly extraordinary thing because an object that's no bigger than the solar system, and in many cases significantly smaller, is making enough power to outshine the surrounding galaxies by more than 1,000 in some cases. That's even more impressive than a tiny little atomic nucleus in an atom producing all the power that it can produce.

• 6:29Regina Barber?

  But how do these jets exist in the first place? Well, to solve any good mystery, we need clues. And we're better to look for clues than the black holes themselves. Black holes have two important clues. First, black holes spin, or as we say in physics, they have angular momentum. But why are black holes spinning?

• 6:52Speaker 02

  We believe that black holes are spinning because of the way in which material accretes onto them.

• 6:59Regina Barber?

  The dust and gas getting sucked up into the black holes, they're also spinning and have angular momentum. But along the way to entering a supermassive black hole, that dust and gas swirling in, it loses angular momentum. They swirl less.

• 7:13Speaker 02

  That's why it's able to swirl all the way in, but there's angular momentum that you get and that spins a black hole. Plus, we also know that's one way black holes grow. So the spin is a consequence, we believe, of how black holes actually assemble and grow in mass.

• 7:34Regina Barber?

  So stuff accumulates around these black holes and causes them to spin. But how is that material getting shot out into space in these huge energetic jets?

• 7:44Regina Barber?

  Clue two: magnetic fields. Black holes have magnetic fields like stars and some planets do, and as black holes spin, these magnetic fields get tangled up. And charged particles in the gas get carried away along field lines and eventually into the jets.

• 8:01Speaker 03

  There is a source of power, which is the spin of the black hole, and the magnetic field is the agency for removing it, if you like. But in addition, there's all this gas that is falling inwards, swirling around the black hole. And just like a satellite in the atmosphere, it gets hot, loses energy, falls down.

• 8:25Regina Barber?

  And then it goes out. And with all these charged particles, you get radiation.

• 8:30Speaker 02

  So by the time I was in graduate school, we already knew that the primary mechanism for jet production in black holes arises from sort of the interaction of strong magnetic fields with the spinning black hole, where the rotation of the black hole kind of twists and amplifies these magnetic field lines and pushing and propelling charged particles along the poles of the black hole into these powerful jets. And this process is often referred to as the Landford-Znajek mechanism.

• 9:01Regina Barber?

  And that's Roger, one of those names.

• 9:03Speaker 02

  Absolutely.

• 9:05Speaker 03

  Yes, so I can talk about that.

• 9:06Regina Barber?

  The Landford-Znajek process is a culmination of all of Roger and Roman's work. And it upends the idea that black holes are just vacuum cleaners.

• 9:15Speaker 03

  The popular view is that you can't get anything out of a black hole. But if the black hole is spinning and there are magnetic field lines, like those you would associate with a picture of a magnet or the earth, if they go through the surface of the black hole, which is called the event horizon, then it is possible to take power out of the black hole. And that comes at the expense of the rotational energy of the black hole.

• 9:47Regina Barber?

  This hypothesis and its equations bring together big concepts in physics, like Albert Einstein's equations of general relativity, how gravity is just a warping of spacetime, and James Clerk Maxwell's older electromagnetism equations that describe how electricity and magnetism interact.

• 10:05Speaker 02

  It's just really beautiful because it combines these two very deep and profound ideas and shows you how energy can actually be extracted.

• 10:13Regina Barber?

  But all of this is really hard to observe. And it wasn't until the last decade or so that scientists were able to really see the jets up close, with a bunch of radio telescopes around the world showing high-resolution images of the jet coming from that supermassive black hole at the center of M87, the one Hebert Curtis stumbled upon in 1918.

• 10:35Regina Barber?

  Images also showed a magnetic field around the black hole, one step in supporting the Blandford-Snye process.

• 10:43Speaker 03

  The fields are very strong and somewhat organized, as might have been expected.

• 10:52Speaker 03

  And also, they look like they're being created by a gas swirling around the black hole. And they have the sort of distortion you might expect with it.

• 11:09Regina Barber?

  These images are the closest we can get to studying black holes directly.

• 11:13Speaker 02

  They're as up close as we can possibly get to a black hole's event horizon. You're able to see where the jet starts. And so that is pretty unprecedented.

• 11:27Regina Barber?

  They're so close that scientists aren't just learning about jets. They're learning about the fascinating characteristics of black holes in general, which as long as scientists have known about them, have been super mysterious.

• 11:39Speaker 03

  Understanding the history of these black holes, how they started, how they grew. Like babies, they are fed and they grow. And how this happened, what their impact was on their surrounding galaxies, how they framed the evolution of the universe, that is a thing we're learning.

• 12:03Regina Barber?

  Everything we're learning about black holes points to them being huge players in the universe. The supermassive black holes in the center of galaxies affect the whole galaxy, the billions of stars in them, and the planets that orbit those stars.

• 12:17Speaker 02

  We now believe that these black hole jets are critical and they sort of modulate the formation of stars in the galaxy itself. So they become sources of energy and produce, they add heat to the gas. They prevent stars from forming by, they prevent cooling. So you don't form stars. So they act as a weird sort of, you know, interestingly, like a piston, if you will, of how many stars will form. And so they appear to be shaping the morphology. So how stars are formed, where they form in galaxies, and what times they form in. So they seem to be really much more important than we previously believed.

• 13:04Regina Barber?

  So, black holes. Destroyers, yeah. But at times, creators too. They're powerful entities molding galaxies from the inside out. And we are just starting to get to know them.

• 13:24Regina Barber?

  Short waivers, we've covered black holes a lot over the years. We'll leave links to those episodes in our show notes in case you want to hear more about them. Also, thank you so much for listening. Make sure you never miss a new episode by following us on whatever podcasting platform you're listening to. This episode was produced by Rachel Carlson and edited by showrunner Rebecca Ramirez. Rachel and Tyler Jones checked the facts. Jimmy Keeley was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Short Wave, the science podcast from NPR.

• 14:19Speaker 00

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